A variety of different dough products are presently available to consumers for production of different sorts of baked stuffs, such as pizza-crusts, buns, croissants, etc. These products may generically be divided into two major groups: 1) those goods based on the process of leavening the dough, namely those goods wherein the dough is leavened by means of chemical agents, and 2) those goods wherein the dough is leavened by the fermentative activity of baker's yeast contained in the dough.
The use of chemicals as the leavening agent for a dough has been commonly used and has the advantage that the chemicals' behavior is based on a predictable chemical reaction, allowing for a specific control of the volume of carbon dioxide produced to leaven the dough. Since the amount of carbon dioxide production and also the moment at which said production is to take place may be controlled, the manufacture of the baked products from the dough can be carried out even after a long shelf life thereof.
The final baked good products obtained therewith are inferior in overall quality as compared to dough products leavened by means of baker's yeast. In particular, the texture of such products is often not acceptable to customers and in addition, the products lack flavor compounds produced by the yeast during its leavening action.
For this reason, producers of edible baked goods try to avoid the use of such chemicals in their products and rely instead on the use of baker's yeast. Unfortunately, products containing ordinary baker's yeast suffer from a variety of different problems inherent to the utilization of “live microorganisms.”
One of these problems is that the activity of yeast in a dough cannot be controlled straightforwardly. For this reason, yeast containing dough compositions may be stored only for a limited period of time because under common conditions of storage, such as room temperature or even lower temperatures (e.g., storage in a refrigerator), and ordinary baker's yeast shows substantial activity resulting in a consistent production of carbon dioxide. This continuous activity of the ordinary baker's yeast beyond the desired degree of proofing negatively affects the organoleptic and Theological properties of the dough, which results in unacceptable final products from the point of view of taste and texture.
One approach taken to avoid this particular setback was to store a yeast containing dough, optionally in pre-baked form, at freezing temperatures of about −20° C., so as to reduce the activity of the yeast to a minimum.
To this end, European Patent No. 0 442 575 instructs dough composition use with the substrate limiting concept. Accordingly, a dough is leavened with a maltase negative yeast until all of the directly fermentable components thereof are consumed, the dough is then frozen for long term storage. Before consumption, the frozen dough is thawed and further leavened by means of chemical agents. This approach also proved to be unsatisfactory as products that are prepared from frozen dough compositions are not as convenient for the consumer as fresh (e.g., refrigerated) dough products. The frozen dough must be thawed and, in most instances, pre-proofed prior to baking. Pre-proofing requires consumer monitoring so as to avoid excessive proofing of the dough.
Moreover, the texture of the final baked products derived from frozen doughs has been shown to be inferior as compared to products produced from non-frozen doughs. In addition, the characteristic flavor associated with yeast leavening is also inferior or often not present at all in final baked products derived from frozen doughs.
Another approach taken to overcome the storage problem of fresh, yeast containing dough compositions was the development and utilization of low temperature inactive (lti) yeast strains in dough. Lti-yeast strains are yeast strains that are essentially inactive at low temperatures, but retain their activity when brought to higher, or proofing, temperatures.
European Patent No. 0 487 878 describes a process for constructing yeast strains with lti-properties, wherein a strain of Saccharomyces cerevisiae is subjected to a mutagenic treatment. The treatment dictates that at least one mutant with an lti-property is selected and backcrossed at least once with a wild type haploid strain of Saccharomyces cerevisiae having an opposite mating type. At least two of the backcross segregants having lti-properties and opposite mating types are then selected and are crossed at least once. This mutagenic treatment results in the selection of a diploid strain having growth potential, an lti-property, and the ability to raise a dough.
Further, the construction of different lti-derivatives have been described. European Patent No. 0 663 441 describes a process for constructing lti-strains that react more sluggishly with the maltose contained in the dough. These sluggish lti-strains may be obtained by first crossing a haploid Saccharomyces cerevisiae that possesses an lti-property with a haploid Saccharomyces cerevisiae strain that has an active maltase gene under catabolic repression, then crossing the segregants, and finally selecting a diploid strain with an lti-property, an active Mal-phenotype (Mal+) expressing the gene coding for maltase either inducibly (wild-type) or constitutively, and growth potential.
From the industrial manufacturer's point of view, one of the major problems encountered in preparing dough compositions or edible baked products therefrom resides in that yeast strains allowing for large scale production at reasonable expenses have to be utilized. The yeast strains needed for industrial scale production comprise of a high yield, a high biomass production, or a good dryability. Presently, there are only a limited number of strains commercially available with such characteristics that enable said dough composition production on an industrial scale. Examples of such strains are Levure Boulangeère Bleue (available from Lesaffre et Cie, Paris, France), Fermipan (Fermipan Red, available from Gist-Brocades, Delft, The Netherlands), or HS (available from Hefe Schweiz, Stettfurt, Switzerland). The genetic reason for this lack of acceptable yeast strains is that the genes which cause these properties are not yet elucidated. The available industrial yeast strains are difficult to modify for new particular properties, such as low temperature inactivity, glucose-de-repression, trehalase deficiency, or one or several auxotrophies.
Therefore, there is a need in the art to enable a skilled person to introduce such properties into the genetic background of industrial bakers' yeast. Since these properties of yeast strains are most often based on a recessive allele, a combination of such a property with the properties of an industrial strain is not an easy task to achieve. The difficulty of the task resides in providing a method to introduce properties of known strains into the genetic background of industrial's baker's yeast, which results in novel yeast strains having both the property afforded by the recessive allele and the properties provided by industrial baker's yeast. Thus, it would be desirable to determine a process by which properties of known yeast strains can be introduced in the genetic background of baker's yeast.